The prior art has employed many devices and systems to process and purify water from industrial operations and municipal sources prior to discharging the water. Activated-sludge wastewater treatment plants (WWTP's), which are well known in the art, have been most often utilized to address this problem. Additionally, many industrial and municipal water treatment plants utilize biological systems to pre-treat their wastes prior to discharging into the usual municipal treatment plant.
Microorganisms used in the activated sludge break down or degrade contaminants for the desired water treatment in these processes. Efficient process performance and control requires quick and accurate assessment of information on the activity of the microorganisms. This has proven to be a difficult task in view of the wide variety of materials and contaminants that typically enter into treatment systems. Also, variations in the quantity of wastewater being treated, such as daily, weekly or seasonal changes, can dramatically change numerous important factors in the treatment process, such as pH, temperature, dissolved oxygen, nutrients and the like, alteration of which can be highly detrimental to proper wastewater treatment. Improperly treated wastewater poses serious human health dangers.
Various biological nutrient removal (BNR) processes are often used in wastewater treatment plants to assist in contaminant degradation. In a typical BNR process, contaminants in the wastewater, such as carbon sources (measured as biochemical oxygen demand or BOD), ammonia, nitrates, phosphates and the like are digested by the activated sludge in anaerobic, anoxic and aerobic (oxic) stages, also known in the art. In the biological treatment process, the wastewater, with or without passing through a preliminary settlement process, is mixed with return activated sludge (RAS) from the final clarifiers. The microorganisms suspended in the wastewater, sometimes hereinafter referred to as "mixed liquor", then flow through the biological treatment process that may include one or all three anaerobic, anoxic and aerobic stages for proper nutrient removal.
As mentioned above, it is important to remove nitrogenous contaminants, which is performed in nitrification/denitrification processes. Biological removal of nitrogenous contaminants from wastewater involves two-step distinctive treatment processes: biological nitrification where ammonia-nitrogen is converted into nitrogen oxides such as nitrate and nitrite and biological denitrification where nitrate and nitrite are converted into nitrogen gas and released into the atmosphere. The following reactions represent the nitrogen removal in a typical wastewater treatment process: ##EQU1##
From the nitrogen removal point of view, denitrification is the process where nitrogen is finally removed from wastewater. In a typical wastewater treatment plant, nitrification is achieved in the aerobic process where oxygen is provided through aeration, and denitrification is achieved in anoxic process where oxygen is limited or absence. To achieve denitrification, either a certain fraction of nitrified mixed liquor is recycled to the front anoxic zone before the wastewater flows to the aerobic zone or the nitrified mixed liquor flows to a downstream anoxic zone before discharge to final clarifiers. A combination of internal recycle to the front anoxic zone and secondary anoxic zone that follows the aerobic zone is also a common practice in achieving total nitrogen removal.
In most wastewater treatment processes, nitrification is achieved in the aerobic zone where carbonaceous contaminants have been fully oxidized. The mixed liquor leaving the aerobic zone contains very limited amounts of carbonaceous contaminants. While this is a desirable treatment result as far as carbonaceous biochemical oxygen demand (C-BOD) removal is concerned, it is not a favorable condition to achieve further denitrification. Referring to reaction (3), denitrification requires not only the absence of oxygen, but also the supply of carbonaceous nutrient. The carbonaceous nutrient can be either biodegradable organic material in the water phase or intracellular stored nutrients inside the microorganisms that perform the denitrification.
When carbonaceous nutrient is severely limited, the microorganisms are in endogenous metabolic state and denitrification activity will be very low compared with the condition where carbonaceous nutrient is not limited. It has been a well known practice to feed the microorganisms carbonaceous nutrient to promote denitrification, especially after the aeration process. Methanol, among other organic compounds such as acetic acid and ethanol, is one common chemical used as a nutrient for the enhancement of denitrification because of its relatively low cost and high energy density. Nonetheless, it is important to monitor and control carbonaceous nutrient enhanced biological denitrification processes to maximize wastewater treatment quality and efficiency.